The present invention relates in general to an infrared light detector, and more particularly, to an infrared detector that is a photoconductive bolometer having good responsivity in the short or mid-wave infrared and also in the long-wave infrared.
When light strikes special types of materials, a voltage may be generated, a change in electrical resistance may occur, or electrons may be ejected from the material surface. As long as the light is present, the condition continues. These special types of materials are often used as optical detectors to detect the characteristics of an optical signal. Two broad classes of infrared optical detectors include photon detectors and thermal detectors. A photon detector generates free electrons when the incident radiation excites electrons from the valence band to the conduction band of the detector material. Therefore, the quantum of light must have sufficient energy to free an electron. The response of such type of detector is thus dependent on the wavelength or frequency of the incident radiation. In the thermal detector, the incident radiation is absorbed by the detector material and is manifested as an increase in vibration of atoms in the material, which is referred to as an increase in temperature.
One common type of photon detector is a photoconductive detector made of a semiconductor material. The change in resistance caused by excitation of electrons generated by the incident radiation is referred as Rpc. Heavily doped n-type or p-type semiconductor materials are frequently used for infrared photoconductive detectors. These photoconductive detectors are usually operated by applying a constant bias across them with a fixed voltage that induces an electric field in the material and then measuring the current flowing in the biasing circuit. Since the bandgaps of infrared photoconductive detectors are relatively small, a substantial number of electrons are thermally excited to the conduction band. A small increase in the detector temperature can in most cases excite an additional number of electrons that substantially exceeds those excited by the radiation. The detector temperature is often controlled within 0.01° C. of the nominal operating temperature or less to keep the thermally excited electrons, or thermal noise, from burying the optical signal.
One of the commonly used thermal detectors is a bolometer, which is also one of the first infrared detectors. The bolometer is a resistor that is thermally isolated and exposed to incident radiation, the bolometer temperature changes in proportion to the amount of incident radiation, and of course, the resistance of the device, RTH, changes as well. The resistance change is measured in the same way as it is measured in a photoconductive detector element, by biasing the bolometer with a voltage and measuring the current through it. Bolometers are now found fairly wide use in thermal imaging. Thermally isolated resistor elements, called microbolometers because their small size, are formed in each cell of an integrated circuit called a focal plane array (FPA). The number of cells (or pixels) can be quite large in a typical imager, up to 100,000 elements or more. Megapixel resolution, available now in visible cameras, is certainly feasible for micro-bolometer FPAs as well. The circuitry in each cell senses the micro-bolometer resistance and outputs that information to the imaging electronics that generates the high resolution image.
A microbolometer device tends to have low signal voltage, and thus the most important figure of merit to consider during design and optimization is responsivity, which depends on bias current, thermal conductance, temperature coefficient of resistance (TCR), and coupling efficiency. The temperature coefficient of resistance (TCR) is critical in bolometer material selection. There has been a large amount of research to find a material with a large and stable temperature coefficient of resistance. Currently, vanadium oxide VO2 and amorphous silicon are the most popular and promising materials for room temperature infrared bolometer applications. Semiconductor bolometers have been developed, but are typically used for sensing micro-wave radiation. The detector described herein is a semiconductor bolometer. The temperature coefficient of resistance (TCR) of vanadium oxide is between about 2.2 and 2.6% ° C.−1. Ideal bolometer performance is limited by thermal noise and has a responsivity proportional to the bias current and the TCR.
The micro-bolometer is a power detector that works well in the low-wave infrared (LWIR) by using fast (that is, low f-stop number) optics viewing terrestrial scenes around 300 K. This is because infrared (IR) energy peaks in this LWIR waveband. For maximum quantum efficiency, the detector is designed with a cavity dimension set to one quarter of the desired peak wavelength. If the detector is designed for maximum efficiency in the LWIR, its out-of-band responsivity will be poor. For terrestrial scenes, infrared energy emitted in the mid-wave infrared (MWIR) is approximately 1/10 of that emitted in the LWIR. For this reason, a micro-bolometer designed for peak response in the MWIR waveband will have a lower response than the LWIR micro-bolometer.
Therefore, the conventional photoconductive infrared detector, though having sufficient responsivity in the MWIR, often requires a cryogenic cooling system to prevent the optical signal from being buried by the thermally excited electrons or noise. The conventional bolometer designed for maximum efficiency in the LWIR normally has poor responsivity in the LWIR. There is thus a substantial need to develop a detector having numerous applications in infrared sensing systems with strict cost, size, weight, and power constraints that sense infrared radiation in both LWIR and either SWIR or MWIR without the requirement of using a cryogenic cooling system.